1. Field of the Invention
The present invention concerns processing artificial persistence or afterglow in a digital image converter. It also concerns devices operating this process.
The essential role of an image converter is to convert an image at relatively slow renewal into a television type image, which is very luminous, allowing its exploitation in bright surroundings.
This slow renewal image is generally a radar image but it can also be images issuing fromaa sonar, an infra red sensor, an echography device, that are desired to be visualized on screens functioning as television receiver screens.
According to the prior art, the image converters initially used memory tubes generally comprising two guns, a writing gun controlled in radar sweeping, for example, and a reading gun controlled in television sweeping. Thereafter, digital image converters D.I.C) were introduced, that used digital circuits.
FIG. 1 represents schematically a digital image converter to which the present invention applies.
Such a converter essentially comprises a circuit 1 forming the radar interface in which the incident video is processed, this circuit receiving the radar video signals VI jointly with radar synchronization signals SY, and a circuit 2 for conversion of the polar coordinates, .theta. of the radar video in Cartesian representation XY. Circuits 1 and 2 are connected to a digital random access memory 3 (RAM) through an addressing circuit 7. Afterglow circuit 4, the object of the invention, is interposed between interface 1 and memory 3. A TV sweeping generator 5 is connected, through the addressing circuit 7, to memory 3. It is also connected to a television monitor 6, which itself is connected to memory 3.
The functions of the DIC above are the following:
Interface circuit 1 samples and puts into digital form the radar video signals VI that are applied to it. It can include a video compression circuit allowing the acquisition of video signals received by the radar after its emission of a pulse for a defined angle of the antenna or aerial in rotation, corresponding to a synchronization pulse SY, and the reading of these video signals, in delayed time and at a different speed, so as to be adapted to the access time of the image memory 3;
circuit 2 for the conversion of the polar coordinates into Cartesian coordinates allows the calculation of the address of each image element in Cartesian coordinates from radar data received in polar coordinates;
the image memory 3 has a capacity adapted to the television standard used. It can have, for example, 1 024 lines of 1.024 memory points. The luminence of each point or dot can be coded, for example with the use of 3 bits, authorizing eight levels of video intensity for each dot. For this memory, the television reading phase and the radar writing phase are asynchronous. Reading has priority and during a reading phase, conversion is interrupted;
circuit 5 carries out the following operations:
generation of television synczhronization signals, PA1 simultaneous reading of several dots in the image memory 3, in such a way as to respect the access time of the circuits used and to allow writing in this single memory; PA1 analog-digital conversion of this video intensity data read in the image memory in order to generate a television video signal for the television monitor 6 on which the visualized data appears.
Afterglow circuit 4 acts to restore, with respect to digital data, for which the afterglow does not exist, an afterglow effect comparable to that produced on an analog memory tube. The recorded data is not attenuted by itself as a function of time in a digital memory that, without this particular process called artificial afterglow, has a tendency to perform indefinitely, the television image tending towards saturation. On the contrary, on a tube, the brightness of a spot starts to drop once it has been recorded. For a digital memory therefore, the afterglow circuit 4 creates a similar effect, sometimes with a delay of one antenna revolution and a decrease of the level quantified at each revolution.
According to the prior art, the artificial afterglow applied to a D.I.C. follows a law of fixed decrease, i.e. the law of composition between the incident video and the video already recorded in the memory, follows a law which is not variable in time. If the incident video is called Vi, issuing from interface 1, the video existing in the memory 3 is Vm, and the resulting video Vr, i.e. that which will be rewritten in the memory 3, the law that follows the artificial afterglow is the following:
If Vi.gtoreq.Vm, Vr=Vi, i.e. if the incident video has an amplitude superior or equal to the video in the memory, for the memory cell involved, the value of the incident amplitude (Vi) is rewritten into the memory as the value of the resulting video.
If Vi&lt;Vm, Vr=Vm-k, i.e. if the amplitude of the incident video is inferior to that of the video in memory, Vm-K is chosen as the amplitude of the video signal to be rewritten in memory, the value of the signal already in the memory Vm decreased by a determined constant value k that is called the "decrement factor".
This operation must occur only once for each antenna revolution. In order to check it, each elementary cell of the memory comprises a bit called an afterglow bit which, at each registration operation, is compared to a "revolution bit" that changes value at each antenna revolution. In the case where the incident video signal Vi has an amplitude inferior to the amplitude of signal Vm already put into memory, the retentivity bit is compared with the revolution bit.
If there is inequality between these two bits, signifying that the decrementation of the video was not yet carried out at the antenna revolution involved, for the memory cell analyzed, the new value of the video signal put in memory is Vr=Vm-k. The afterglow bit is thus changed, assuming its other logic value, allowing to avoid subsequently at the same antenna revolution a further decrementation for that cell.
If there is egality of the afterglow bit and of the revolution bit, this means that operation Vm-k has already been carried out during the antenna revolution involved for the cell concerned and it will not be repeated.
The decrementation process being carried out simultaneously with the write operations, i.e. in synchronization with the rotation of the antenna for a radar (in a more general framework it would be in synchronization with the arrival of the data from position of a sensor), the effect obtained on the image is very close to that obtained with an analog image converter. On these latter, the signal decreases once it is written, whereas in a digital image converter the decrease begins at the following antanna rotation with a decrementation quantum at each rotation.
FIG. 2 shows the evolution of the brightness level of a given dot, supposed to be initially at the maximum level equal to 7, in relation with the number of antenna revolutions, thus taking into account the evolution of the afterglow in the cases of an analog image (curve 1) and digital (curve II) converter. In the second point, it will be quantified, with k=1.
It will also be noted that the term "k" allows fixing of the decrease law of the video in memory. For a video expressed on n bits, the video signal disappearance occurs after 2.sup.n -1 antenna rotations if k=1.
FIG. 3 is analogous to FIG. 2 and shows that for k=2 and a video signal expressed on eight levels, the decrease is obtained on four rotations.
According to the prior art, it is also possible to vary the term k from 1 to 2.sup.n for a video expressed on n bits. However, for a given equipment, the term k once selected remains constant.
It can also be noted that for certain values of k, the time necessary for the disappearance of the data does not follow a regular law as a function of the initial level of the incident video.
FIG. 4 is analogous to FIG. 2 and represents a decrease example of a video signal expressed on eight levels with k=4.
It will be observed that a video data entered with levels 7, 6 and 5 remains in memory during two antenna rotations, while the data entered with levels 4, 3, 2 or 1 only remains there during one rotation.
Thus, according to what is set out herein-above, it is possible to obtain decrease laws of video data that are close to the natural decrease observed in the analog image converters. However, the decrease law that was selected for a converter remains fixed with time.
It can, however, be selected to be more or less rapid and in this case, it has been observed that for certain relatively high values of k, the decrease of brightness of the data was not regular, the high level of data remaining in the memory longer than the low level data. Furthermore, all the data, whatever their input level, are processed in the same way.
There are, however, certain cases where it would be worthwhile modifying, in a more selective manner, the decrease law of the afterglow in a digital image converter with the purpose of obtaining an improved exploitation of the data displayed on the screen of the television monitor. Thus, in the case where the radar data are relative to afterglow slow moving targets, like ships, it is difficult, when a DIC is used for which the decrease of the afterglow is adjusted according to the prior art, to separate the last echo received from the preceding ones (it is the phenomenon called "fusion"). As a matter of fact, such a target is almost at the same place from one antenna revolution to the next; thus, the echos of that target are displayed substantially on the same area on the TV monitor, producing a large spot. In that case, it is very difficult for the observer to separate the last echo from the spot, and the data concerning the trajectory and the direction of the target are lost. Further, prior art devices are not suitable for marine surroundings due to sea clutter, on which, weak echoes are difficult to select, and also due to some particular fixed targets like a buoy which give rise to a fixed echo but which can be sometimes invisible due to waves.
It is an object of the invention to provide a processing of the afterglow in a digital image converter so that the decrease law that is applied is variable with time and adaptable to the surroundings.